Explain enable-features not required for use_asan_unowned_ptr=true

Clarify docs/dangling_ptr.md for this case.

Change-Id: Ic2588335456abbac78e71cb445a948640475f495
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4245828
Reviewed-by: Bartek Nowierski <[email protected]>
Commit-Queue: Tom Sepez <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1104855}
diff --git a/docs/dangling_ptr.md b/docs/dangling_ptr.md
index c16f51c..8ae4cd4 100644
--- a/docs/dangling_ptr.md
+++ b/docs/dangling_ptr.md
@@ -150,3 +150,6 @@
 This will crash when the object containing the dangling ptr is destructed,
 giving the usual three-stack trace from ASAN showing where the deleted object
 was allocated and freed.
+
+When running under this mode, there is no need to specify any --enable-features
+flag as above.